DRAM cell structure with buried surrounding capacitor and process for manufacturing the same

ABSTRACT

A memory device that includes a semiconductor substrate, and an array of memory cells, each cell being electrically isolated from adjacent cells and including an island formed from the substrate, the island having a top portion and at least one sidewall portion, and being spaced apart from other islands by a bottom surface on the substrate, a capacitor formed contiguous with the sidewall portion, and a transistor formed on the top portion of the island, the transistor including a gate oxide layer formed on a surface of the top portion, a gate formed on the gate oxide layer, and a first and a second diffused regions formed in the top portion, the first diffused region being spaced apart from the second diffused region.

FIELD OF THE INVENTION

This invention pertains in general to semiconductor devices, and, moreparticularly, to memory cells for a dynamic random access memory (DRAM)and processes for manufacturing the same.

BACKGROUND OF THE INVENTION

In the semiconductor industry, DRAMs are among the most importantintegrated circuits and the source of continuing research anddevelopment. There is a continuing effort to increase their storagecapacity, improve writing and reading speed, and decrease devicedimensions. A DRAM cell generally includes a transistor and a capacitoroperated by the transistor. Conventionally, DRAM cell designs can bedivided into three types, namely planar, stacked-capacitor and trench.In the planar design, the transistor and capacitor of a cell areproduced as planar components. In the stacked-capacitor design, thecapacitor of a cell is disposed above the transistor. In the trenchdesign, the transistor is disposed on the surface of a substrate, andthe capacitor is disposed in a trench formed in the substrate.

The process of forming a trench, however, requires an accurate alignmentof mask work. For deep sub-micron semiconductor devices, a deep trenchmay have a length-to-diameter aspect ratio of 40:1. Typically,capacitors are formed in the deep and narrow trenches by depositing adielectric layer on the trench walls and filling the trench with a dopedpolysilicon layer. As the aspect ratio becomes higher, for example,exceeds 20:1, the trench becomes more difficult to fill.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to DRAM cells thatobviate one or more of the problems due to limitations and disadvantagesof the related art. The present invention also provides processes formanufacturing the cells.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the cell structures particularly pointed out in the writtendescription and claims thereof, as well as the appended drawings. Toachieve these and other advantages, and in accordance with the purposeof the invention as embodied and broadly described, there is provided amemory cell that includes a semiconductor substrate, a capacitor and atransistor, in which the substrate includes an island formed from thesubstrate, the island having a top portion and at least one sidewallportion, the capacitor is formed contiguous with the sidewall portion,and the transistor is formed on the top portion of the island, thetransistor including a gate oxide layer formed on a surface of the topportion, a gate formed on the gate oxide layer, and a first and a seconddiffused regions formed in the top portion, the first diffused regionbeing spaced apart from the second diffused region.

In one aspect of the present invention, the capacitor includes a dopedregion formed in the sidewall portion, a dielectric layer formedcontiguous with the sidewall portion, and a polysilicon layer formedcontiguous with the dielectric layer.

In another aspect of the present invention, the memory cell includes aburied strap that couples the first diffused region of the transistor tothe second plate of the capacitor.

Also in accordance with the present invention, there is provided amemory device that includes a semiconductor substrate and an array ofmemory cells, each cell being electrically isolated from adjacent cellsand including an island formed from the substrate, a capacitor and atransistor, in which the island has a top portion and at least onesidewall portion, and is spaced apart from other islands by a bottomsurface on the substrate, the capacitor is formed contiguous with thesidewall portion and the transistor is formed on the top portion of theisland, the transistor including a gate oxide layer formed on a surfaceof the top portion, a gate formed on the gate oxide layer, and a firstand a second diffused regions formed in the top portion, the firstdiffused region being spaced apart from the second diffused region.

Still in accordance with the present invention, there is provided aprocess for manufacturing a memory device, the process includingdefining a semiconductor substrate, etching the substrate to form anarray of islands spaced apart from one another by a bottom surface onthe substrate, each island having a top portion and at least onesidewall portions, doping the sidewall portion and the bottom surface,forming a dielectric layer contiguous with the sidewall portion, forminga polysilicon layer contiguous with the dielectric layer, electricallyisolating the array of islands with an insulating material, forming anoxide layer on a surface of the top portion, forming a gate on the oxidelayer, and forming a first diffused region and a second diffused regionspaced apart from the first diffused region in the top portion.

In one aspect of the present invention, the process further includesetching the insulating material disposed on the bottom surface in adirection of a bit line to expose the polysilicon layer, depositing aconductor layer on the etched insulating material, and etching theconductor layer to form a first conductor portion and a second conductorportion spaced apart from the first conductor portion, in which thefirst conductor portion couples the first diffused region to thepolysilicon layer.

In another aspect of the present invention, the process further includesetching the insulating material disposed on the bottom surface in thedirection of the bit line to expose the second diffused region, anddepositing a contact layer on the etched insulating material to couplethe second diffused region to the bit line.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention and,together with the description, serve to explain the objects, advantages,and principles of the invention.

In the drawings,

FIG. 1A shows a top view of a memory cell array layout consistent withthe present invention;

FIG. 1B shows a perspective view of a memory cell array consistent withthe present invention;

FIG. 1C shows a top view of a single memory cell layout consistent withthe present invention;

FIG. 2 shows a perspective view of a memory cell consistent with thepresent invention;

FIG. 3 shows a top view of the layout of buried straps and bit contactsin a cell array;

FIG. 4 is a schematic of a memory cell consistent with the invention;and

FIGS. 5A to 5E show the process for manufacturing a memory cell arrayconsistent with the invention.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the present embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

FIG. 1A shows a top view of a layout of a memory cell array 10 inaccordance with the invention. Referring to FIG. 1, the cell array 10includes a substrate 12 and a plurality of memory cells 14 formed on thesubstrate 12. A representative cell 14A includes a three-dimensionalisland 20, a capacitor 30 surrounding the island 20 and a transistor(not shown) formed on the island 20. The island 20 is a part of thesubstrate 12 and, in the embodiment, has a substantially rectangularcross section. Other embodiments may include islands in the form of acylinder. In the cell array 10, a trench space 18 is defined as thespace between the areas defined by opposing sidewalls of two adjacentmemory cells.

FIG. 1B shows a perspective view of the cell array 10 prior to theformation of transistors. Referring to FIG. 1B, each island 20 includesfour sidewall portions 22 wherein a capacitor 30 is formed and a topportion 24 where a transistor (not shown) will be formed. Each island 20is spaced apart from adjacent islands by a bottom surface 16 on thesubstrate 12. The trench space 18 is thus defined between two adjacentislands by their opposing sidewall portions 22 and the bottom surface 16therebetween.

The capacitor 30 includes a first plate 32 formed within the sidewallportion 22, a dielectric layer 34 formed contiguous with the sidewallportion 22, and a second plate 36 formed contiguous with the dielectriclayer 34. In one embodiment of the invention, the first plate 32 is adoped silicon region, the dielectric layer 34 is a nitride oxide layerand the second plate 36 is a polysilicon layer.

FIG. 1C shows a top view of the layout of the representative cell 14Ashown in FIG. 1A and, in particular, the first plate 32, the dielectriclayer 34 and the second plate 36 of the capacitor 30. In addition, FIG.1C shows an insulating material 38 filling the trench spaces 18 andremaining spaces among the islands 20 to electrically isolate the cells.In one embodiment, the insulating material 38 is oxide.

FIG. 2 shows a perspective view of the representative cell 14A shown inFIG. 1A. Referring to FIG. 2, a transistor 40 is formed on the topportion 24 of the island 20 and includes a gate oxide layer 42 disposedon a top surface 240 of the top portion 24, a gate 44 formed on the gateoxide layer 42, and a pair of spaced-apart diffused regions 46 formed inthe top portion 24. The gate 44 is coupled to a word line (not shown).The diffused regions 46 serve as a source/drain pair for the transistor40, and one of the diffused regions 46 is coupled to a bit line (notshown).

A collar oxide layer 48 may be disposed on the upper part of thesidewall portion 22 and contiguous with the dielectric layer 34 and thesecond plate 36 to reduce parasitic leakage at the sidewall portion 22.A buried strap 50 couplestone of the diffused regions 46 of thetransistor 40 to the second plate 36 of the capacitor 30. The buriedstrap 50 provides a path for the transistor 40 to read from or writeinto the second plate 36 of the capacitor 30. A bit line contact layer60 is disposed between two adjacent cells and adjoins the opposingdiffused regions 46 and 46′ of the adjacent cells. The bit line contactlayer 60 couples the opposing diffused regions 46 and 46′ to a bit line.

FIG. 3 shows a top view of the layout of buried straps 50 and bit linecontacts 60 in the cell array 10. Referring to FIG. 3, as an example ofthe cell 14C, in a bit line (BL) direction, one of the diffused regions(not shown) of the cell 14C is coupled to a buried strap 50C, and theother diffused region is coupled to a bit line contact 60C. The buriedstrap 50C of the cell 14C is spaced laterally from the buried strap 50Dof an adjacent cell 14D, and the bit line contact 60C is shared by acell 14B and the cell 14C. The gate (not shown) of the cell 14C iscoupled to a word line (WL).

FIG. 4 shows a schematic view of a single cell 14. The gate 44 of thetransistor 40 on the top portion 24 is coupled to a word line (WL). Onediffused region 46 of the transistor 40 is coupled to a contact layer(CB) and in turn to a bit line (BL). The other diffused region 46 of thetransistor 40 is coupled to the second plate 36 (not shown in FIG. 4) ofthe capacitor 30 by a buried strap (BS). The first plate 32 of thecapacitor 30 is coupled to the substrate 12, or a reference voltageVsub.

FIGS. 5A to 5E show the process for manufacturing a memory cell array 10in accordance with the invention. Referring to FIG. 5A, the processbegins with preparing a substrate 12, which may be a silicon substrate,an silicon on insulator (SOI) substrate, or a gallium arsenidesubstrate. The substrate 12 can be undoped, lightly doped or heavilydoped with dopants. In the embodiment, the substrate 12 includes a bulkportion that is advantageously a p-type monocrystalline silicon. Thesubstrate 12 is cleaned to remove contaminants and then etched to forman array of three-dimensional islands 20. Each island 20, which is apart of the substrate 12, includes a top portion 24 and sidewallportions 22, and is spaced apart from adjacent islands 20 by a bottomsurface 16 on the etched substrate 12. The opposing sidewall portions 22of two adjacent islands 20 and the bottom surface therebetween togetherdefine a trench space 18. Since the process of the invention does notrequire any accurate mask work in defining a trench, the invention isgenerally immune to the mask misalignment and high aspect ratiolimitations found in conventional techniques.

Next, the sidewall portions 22 and the bottom surface 16 are doped withN+ type dopants such as As by, for example, a drive-in process. The N+doped region 32 corresponds to the first plate of the capacitor 30. Thefirst plate 32 is coupled to the substrate reference voltage (notshown). Subsequent to the formation of the first plate 32, a dielectriclayer 34 such as nitride oxide (NO) is deposited on the sidewallportions 22. A polysilicon layer 36 is then deposited on the dielectriclayer 34. The polysilicon layer 36 corresponds to the second plate, orstorage node, of the capacitor 30.

Referring to FIG. 5B, an insulating material 38 such as oxide isdeposited on the bottom surface 16 to fill the spaces, including thedefined trench spaces 18, among the islands 20. A chemical mechanicalpolarization (CMP) process is then performed to polish the layer ofinsulating material 38 down to the top surface of the islands 20. Theinsulating material 38 electrically isolates the cells.

Referring now to FIG. 5C, gate oxide layers 42 and gates 44 are formedon a top surface 240 of the top portion 24 by conventional processes.The diffused regions 46 corresponding to the source/drain pair of thetransistors 40 are then formed by ion implantation.

FIG. 5D shows the formation of a buried strap 50 to couple thepolysilicon layer 36 of the capacitor 30 to one of the diffused region46 of the transistor 40. The insulating material 38 disposed on everyother bottom surface in the direction of a bit line is etched to exposethe polysilicon layers 36 of two adjacent cells. A conductor layer isdeposited on the etched insulating material 38′. The conductor layer isetched to form a first conductor portion 502 and a second conductorportion 504 spaced apart from the first conductor portion 502. The firstconductor portion 502 couples the diffused region 46 of a cell to thepolysilicon layer 36 of the cell, and the second conductor portion 504couples the diffused region 46′ of the adjacent cell to a polysiliconlayer 36′ of the adjacent cell.

FIG. 5E shows the formation of a bit line contact layer 60 to couple thediffused regions to a bit line. The remaining insulating material 38disposed on every other bottom surface in the direction of the bit lineis etched to expose the remaining diffused regions 46. A contact layer60 is then deposited on the etched remaining insulating material 38″ tocouple the diffused regions 46. A passivation layer 70 such as BPSG(borophosphosilicate glass) is then deposited to provide insulation andprotection for the cells.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the disclosed processwithout departing from the scope or spirit of the invention. Otherembodiments of the invention will be apparent to those skilled in theart from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

1. A memory cell, comprising: a semiconductor substrate, including anisland formed from the substrate having a top portion and at least onesidewall portion; a capacitor formed contiguous with the sidewallportion; and a transistor formed on the top portion of the island, thetransistor including a gate oxide layer formed on a surface of the topportion, a gate formed on the gate oxide layer, and a first and a seconddiffused regions formed in the top portion, the first diffused regionbeing spaced apart from the second diffused region.
 2. The memory cellof claim 1, wherein the capacitor includes a first plate formed in thesidewall portion, a dielectric layer formed on a surface of the sidewallportion, and a second plate formed contiguous with the dielectric layer.3. The memory cell of claim 2, wherein the first plate is doped silicon.4. The memory cell of claim 2, wherein the second plate is a polysiliconlayer.
 5. The memory cell of claim 2, further comprising a collar oxidelayer disposed on an upper sidewall portion and contiguous with thedielectric layer and the second plate.
 6. The memory cell of claim 1,further comprising a buried strap to couple the first diffused region ofthe transistor to the second plate of the capacitor.
 7. The memory cellof claim 6, further comprising a contact layer to couple the seconddiffused region of the transistor to a bit line.
 8. The memory cell ofclaim 1, wherein the gate of the transistor is coupled to a word line.9. A memory device, comprising: a semiconductor substrate; and an arrayof memory cells, each cell being electrically isolated from adjacentcells and including an island formed from the substrate having a topportion and at least one sidewall portion, and being spaced apart fromother islands by a bottom surface on the substrate; a capacitor formedcontiguous with the sidewall portion; and a transistor formed on the topportion of the island, the transistor including a gate oxide layerformed on a surface of the top portion, a gate formed on the gate oxidelayer, and a first and a second diffused regions formed in the topportion, the first diffused region being spaced apart from the seconddiffused region.
 10. The memory device of claim 9, wherein the capacitorincludes a first plate formed in the sidewall portion, a dielectriclayer formed on a surface of the sidewall portion, and a second plateformed contiguous with the dielectric layer.
 11. The memory device ofclaim 10, wherein the first plate is doped silicon.
 12. The memorydevice of claim 10, wherein the second plate is a polysilicon layer. 13.The memory device of claim 10, further comprising a collar oxide layerdisposed on an upper sidewall portion and contiguous with the dielectriclayer and the second plate.
 14. The memory device of claim 9, furthercomprising a buried strap to couple the first diffused region of thetransistor to the second plate of the capacitor.
 15. The memory deviceof claim 14, wherein the buried strap of a cell is spaced laterally fromthe buried strap of an adjacent cell. 16-24. (canceled)